Organic EL elements used in organic EL devices are semiconductor elements that convert electrical energy into optical energy. Recently, research using organic EL elements has been conducted at an accelerated pace. In the field of illumination, this research has already started to clarify issues with putting organic EL devices into practical use. Improved materials, such as an organic material that may form an organic EL element, achieve dramatically lowered driving voltages and increased luminance efficiency. Also, in the display market, televisions using organic EL elements for display screens have come to market.
An increase in current density is attained by impressing a strong electric field so as to obtain a high brightness. It is known that this increase in current density normally generates heat, resulting in acceleration of deterioration of an organic thin film.
An organic EL element has two or more electrodes (a first electrode layer and a second electrode layer) to apply a voltage to the element, at least one of which uses a light-transmissive conductive material, so that when a voltage is applied to the element light generated within the element is emitted outside. Such light-transmissive conductive materials include an extremely thin film made of metal such as Ag or Au, and metal oxide such as indium-doped tin oxide or aluminum-doped zinc oxide. These materials have higher resistance than a metal electrode layer, which normally requires no light transmittance. The higher resistance causes heat generation during power distribution, further resulting in many problems such as reduced luminance efficiency and expanded brightness distribution, in addition to the deterioration described above.
These problems are especially severe in application of surface-emitting organic EL devices to large-area illumination. Some ingenuity is required so as to obtain satisfactory performance. An effective means for solving these problems would be to increase the driving voltage without changing the driving current per unit area.
An EL element forms a p-n junction and emits light by injection of electrons and holes and recombination of those electrons and holes within the element by application of a forward voltage. Formation of a multi-junction connected in series by lamination of a plurality of such junctions in a film-thickness direction increases the driving voltage and further improves the light-emitting brightness without changing the driving current. The patent document 1 specified below discloses a laminated organic light-emitting element, which contains an electrically-insulated charge generation layer between laminated light-emitting units, the charge generation layer being contactless with two-layered internal electrodes and having a specific resistance of 1.0×105 Ω·cm or more. The electrically-insulated thin film simultaneously generates holes and electrons, which are respectively injectable in a hole transport layer and an electron transport layer by application of an electric field to the laminated organic light-emitting element, thereby being applicable to a technique for serially connecting a plurality of light-emitting units (Multi-Photon Emission).
This technique is quite effective for a certain level of area enlargement, but has a problem in that the lamination involves an increased number of man-hours and materials, and further, too much lamination leads to a reduced brightness or luminance efficiency due to absorption of light by the resulting laminated body. Basically, this technique is characterized in that it obtains a high brightness by an equivalent driving current, but because it does not reduce an absolute value of driving current density, it is limited as a means to enlarge the area.
Another means for increasing the driving voltage without changing the driving current per unit area is to divide an element in a film-surface direction and electrically connect the divided elements in series. Several techniques are studied using this means. The patent document 2 specified below discloses a method for producing an organic EL device, in which a patterned lower electrode and a patterned upper electrode are electrically connected in series. This method may achieve the desired purpose to some extent, but is mostly premised on a mask process, resulting in a limitation in area enlargement and problems of complicated procedures and greater loss in the effective area. Still another and similar technique for utilizing a rear cover is disclosed in the patent document 3 specified below, but this technique has essentially the same problems as those in patent document 2.
Patent Documents
Patent Document 1: JP 3933591 B
Patent Document 2: JP 2006-511073 A
Patent Document 3: JP 2008-508673 A